Currently, in the field of electronics devices, research and development of an organic electroluminescence (EL) dye have been intensively done aiming for a next-generation light-emitting material taking the place of a liquid crystal. Theoretically, the organic EL dye can emit light with small electric power, because the dye molecule itself can emit light by feeding a current through a thin film comprising a molecular assembly of the dye. Thus, researches for application to a monitor for electro devices and commercialization are pursued at a high pace aimed at taking the place of a liquid crystal display of high power consumption.
As for the structure of the organic EL device, an organic hole transport layer and an organic electron transport layer are retained between an anode and a cathode, and an organic luminescent dye is contained in the organic hole transport layer and/or the organic electron transport layer, and upon injecting an electron from electrodes into the organic hole transport layer and the organic electron transport layer, respectively, a hole and an electron being combined, and light is emitted when the dye is relaxed from an excited state to a ground state. Therefore, by forming a panel having a multi-layer structure in which a function of carrier-mediated transport and a function of light emission are separated, for example, a two-layer structure of a hole transport layer including a hole-transporting material and an emission layer described in, for example, C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett., 51, (12), 21, Sep. 1987, 913-915, or a three-layer structure in which an electron transport layer including an electron-transporting material is added to the above two-layer structure, the efficiency of recombination of the hole and the electron is enhanced and light is emitted at a low voltage of the order of 10 V. However, in the case of such an organic EL device, since light is predominantly emitted at an interface between the hole transport layer and the emission layer, which is an electron transport layer, by its function as shown in FIG. 6, there is a problem that sufficient emission efficiency can not obtained.
Further, when a multi-layer structure is employed as a device structure, it is necessary to precisely control a film thickness in a plurality of film formation steps and form a pinhole-free film, and therefore there is a problem that time and cost required for producing a device increase.
For this problem, a device structure having a single-layer structure can provide a possibility that the device can be manufactured at a lower cost. Therefore, as a device of a single-layer structure, for example, an organic EL device using an organic compound having both of a hole-transporting ability and an electron-transporting ability is proposed in Japanese Laid-Open Patent Publication No. 2-210790, but a driving voltage is as high as 20 V and emission brightness is not enough.